1. Field of the Invention
The present invention relates to a contact type charging device, particularly to a charging device suitable for image forming apparatuses such as a copying machine and a printer.
2. Related Background Art
In an image forming apparatus using an electrophotographic system, or an electrostatic recording system, a charging device is used for charging image bearing members such as a photosensitive member.
This image forming apparatus usually has the following means constitution and image forming process. Specifically, the process comprises: using the electrophotographic photosensitive member usually of a rotary drum type as the image bearing member, such as selenium, cadmium sulfide, zinc oxide, amorphous silicon, and organic photoconductor; uniformly charging the surface of the photosensitive member to provide a predetermined polarity and potential by charging means; exposing an image on the surface of the charged photosensitive member by image exposure means to form an electrostatic latent image in accordance with the exposed image; attaching a developer toner to the electrostatic latent image by developing means to develop a toner image; and transferring the toner image to a transfer material. The transfer material with the toner image transferred thereon is subjected to a fixing processing by fixing means and discharged as an image forming material (copy, print). Moreover, after the toner image is transferred to the transfer material, the untransferred residual toner is removed from the rotating photosensitive member by cleaning means (cleaner), and the photosensitive member is cleaned and repeatedly used for the image formation.
As the charging means of the photosensitive member as the image bearing member which is a charged member, the charging means of a "corona charging system" using a corona discharge unit has heretofore been used, but in recent years, a "contact charging system (direct charging system)" has been used because the system has advantages such as lower ozone and lower power as compared with the corona charging system.
The contact charging system uses no corona charging, and comprises: placing a conductive member with an adjusted resistance value as the charging member into contact with the photosensitive member as the charged member; applying a voltage (charging bias) to the charging member; and charging the surface of the photosensitive member to provide a predetermined polarity or potential.
As the charging member, in addition to a conductive elastic roller type (charging roller), a conductive elastic blade type (charging blade), a magnetic brush type (magnetic brush member), a fur brush type (fur brush member), and other various types are used.
As compared with the corona charging system, the contact charging system can lower the applied voltage, remarkably reduces the amount of corona products such as ozone, and has other advantages such as a good power efficiency.
In the contact charging system, however, when the charging member is left in contact with the charged member for a long period, the pressure contact surface has a local fatigue deformation.
The charging member is always urged onto the charged member surface by an urging member and its own weight. When the device is left as it is for a long period (e.g., one year or longer) without moving/driving the surface of the charged member, the charging member portion urged onto the charged member surface causes an irreversible deformation (fatigue deformation).
Once such deformation occurs, a charging defect is generated in the portion, and a normal image cannot be obtained.
Moreover, when the sufficient nip width of the charging member and the charged member is obtained to provide a higher charging property, the hardness of the charging member needs to be lowered. Conversely, when the hardness is lowered, the fatigue deformation easily occurs. Even when the period in which no surface moving/driving is performed is not very long, the above-described problem easily arises.
To solve the problem, it is proposed in U.S. Pat. No. 5,095,335 that a charging roller be contacted to or separated from a photosensitive member in association with the mounting/dismounting operation of a process cartridge to an apparatus main body.
Thereby, even when the process cartridge is stored for a long period, the charging roller cannot be deformed.
The contact/separation of the charging roller to/from the photosensitive member is effective for preventing the charging roller from being deformed, but if the charging roller does not exactly contact the photosensitive member, a charging defect is generated.